1. Field of the Invention
The present invention relates to dispersion compensating technique in a ultra-high-speed optical communication system and, more particularly, to a variable dispersion compensator having a chirped grating for changing chirp rate to control group delay time.
2. Description of the Background Art
In an optical communication system using an optical fiber cable as a signal transmission path, a signal is distorted, since an optical pulse is distorted by wavelength dispersion (hereinafter referred to as xe2x80x9cdispersionxe2x80x9d) of the optical fiber transmission path. This is because group velocity is different between wave packets of optical pulses having different wavelengths from each other. That is the group delay time, i.e., time required for a wave packet of an optical pulse to propagate a predetermined length. Ratio of the group delay time to a wavelength is defined as dispersion. In a single-mode fiber (SMF) used as a general optical fiber transmission path, dispersion per 1-km optical fiber transmission path at wavelength of about 1,550 nm has a value of about 16 ps/(nmxc2x7km). This means that difference in a single-mode fiber of length of 1 km is 16 ps between group delay times required for propagating optical pulses having wavelengths different by 1 nm. For example, difference between group delay times is 100 times the above difference, i.e., 1,600 ps, when optical pulses having wavelengths different by 1 nm propagate in an optical fiber having a length of 100 km.
On the other hand, a modulated optical pulse has a spread of line spectra determined by modulation method and bit rate, and its envelope is of a Gaussian distribution type. For example, in return-to-zero (hereinafter referred to as RZ) modulation method, an interval between respective line spectra is 0.08 nm when bit rate (transmission rate) is 10 Gbit/s. The interval is 0.32 nm when the bit rate is 40 Gbit/s. More specifically, the spread of line spectra increases in proportion to the bit rate. Non-return-to-zero (hereinafter referred to as NRZ) modulation method obtains a spread of line spectra which is half the spread of line spectra obtained in the RZ modulation. In this manner, the interval of line spectra which are components of an optical pulse increases as the bit rate increases. For this reason, a difference between group delay times obtained increases, and distortion of the optical pulses increases, when optical pulses are propagated in an optical fiber transmission path. An influence of dispersion of an optical fiber transmission path on an optical pulse increases in proportion to the square of bit rate. In dispersion compensation technique, a device having dispersion which cancels dispersion in an optical fiber transmission path is inserted into the transmission path to approximate the whole dispersion to zero. In particular, the dispersion of a transmission path must be accurately approximated to zero at bit rate of 40 Gbit/s or more.
A variable dispersion compensator using a chirped grating is known as a device which compensates dispersion. For example, such a variable dispersion compensator was proposed by the present inventors in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 8, pp. 827 to 829 (issued in August, 2001). In this variable dispersion compensator, chirped gratings are arranged on 32 thin film heaters, the temperatures of the 32 thin film heaters are independently controlled to apply linear temperature gradient to the chirped grating, thereby making the dispersion of the chirped gratings variable. The present inventors set temperature gradient which linearly is changed from 0xc2x0 C. to 60xc2x0 C. to realize a variable dispersion equalizer having dispersion variable width of 100 ps/nm or more, so that optical signal transmission at 40 Gbit/s is performed. In addition, a chirped grating having a grating length of 40 mm is used.
As described above, a variable dispersion compensator which applies a temperature distribution changing on the basis of a predetermined function to a chirped grating and controls the temperature distribution to control group delay time is useful as a device which can easily variably control dispersion. In the above variable dispersion compensator, the temperature is linearly changed, and the temperature distribution, in which temperature difference between the maximum temperature and the minimum temperature is 60xc2x0 C., is applied to an whole grating, so that a dispersion variable width of 100 ps/nm is realized.
However, when the temperature distribution based on the same temperature distribution function is applied to the whole chirped grating, power consumption disadvantageously increases.
It is, therefore, an object of the present invention to provide a variable dispersion compensator which reduce a power consumption while suppressing deterioration of optical signal characteristics.
In accordance with one aspect of the present invention, there is provided a variable dispersion compensator including an optical waveguide, a temperature controller. The optical waveguide has a chirped grating having Bragg wavelength changed along a longitudinal direction of the grating. The temperature controller controls temperature of the chirped grating. Then, temperature distribution based on a first function T1(x) of the distance x is applied to a central portion of the grating. The central portion is defined as a region where a distance x from an end of the grating is a range of 20% to 80% of total length of the grating along the longitudinal direction thereof. Temperature distribution based on second and third functions T2(x) and T3(x) of the distance x are applied to both end portions of the grating. The both end portions are defined as two regions respectively extending from both ends of the grating to the central region, respectively. At least one of the second and third functions T2(x) and T3(x) has distance dependence different from that of the first function T1(x).
In another aspect of the present invention, there is provided a polarization mode dispersion compensator including a polarized wave separator/synthesizer, a first optical waveguide, a first temperature controller, a second optical waveguide, and a second temperature controller. The polarized wave separator/synthesizer separates light into first and second polarized light components serving as two linear polarized light components, and synthesizes the first and second polarized light components. The first optical waveguide has a first chirped grating which receives the first polarized light component as an input light component and in which Bragg wavelength is changed along a longitudinal direction of the grating. The first temperature controller controls temperature of the first chirped grating. The second optical waveguide has a second chirped grating which receives the second polarized light component as input light component and in which Bragg wavelength is changed along the longitudinal direction of the grating. The second temperature controller controls temperature of the second chirped grating. Temperature distribution based on the first and second functions T1(x) and T2(x) of the distance x are applied to central portions of the first and second gratings defined as a regions where distance x from an end of the first and second grating is a range of 20% to 80% of length of the first and second grating along the longitudinal direction thereof, respectively. The second function T2(x) has distance dependence different from that of the T1(x).
According to the present invention, the temperature at the end portion on the high temperature side can be controlled to a lower temperature. In this case, eye opening penalty which is almost equal to that obtained when temperature distribution based on the same temperature distribution function is applied to the whole region of the grating can be obtained. For this reason, power consumption can be reduced without changing eye opening penalty.